1. Field of the Invention
The invention relates to a belt tightening drive for tightening a seat belt that can be wound about a winding shaft.
2. Background of the Invention
The current state of knowledge is as follows.
Such a reversible belt tightening drive is known for example from DE 10 2008 048 339 A1, in which an electric engine drives a spindle shaft via a first transmission, embodied as a toothed gear transmission, which in turn via a second transmission drives a driven wheel arranged in a torque-proof fashion on a winding shaft in order to wind up a seat belt. This second transmission is formed by a worm, arranged on a spindle shaft, and diagonal external teeth of the driven wheel.
A reversible belt tightening device is activated when needed, here for example in case of a minor crash or a threatened crash, during cross-country driving, or when strong decelerations occur from braking the vehicle, causing the seat belt to tighten. In order to maintain the tight status of the belt in such situations here an electric engine is supplied with high amperage, stressing the vehicle energy system to an undesired high extent.
In case of a crash the triggering of the belt tightening device occurs by way of pyrotechnics and represents a full load tightening, which leads to a decoupling of the belt tightening device from the electric engine. The subsequent holding of the seat belt via the electric drive by a residual current of the electric engine leads to it once more being coupled to the belt tightening device and here disruptions may occur regarding the limitation of force. This residual power supply of the electric engine also strains the vehicle energy system.
Additionally, highly dynamic load conditions occur during full load tightening. They lead to high torque (holding moments) in the driven step of the seat belt tightening device and thus to high gearing forces, particularly in case of a diagonal gearing of the toothed components used, and also to high axial forces, which cause a deformation of the toothed components, such as the wheel body and the bearing points inside the housing of the belt tightening device and thus they may lead to a loss of stability, and ultimately to damages and mechanic failure of the toothed parts.
The objective of the invention is to provide a belt tightening drive of the type mentioned at the outset in which high passive loads, which exceed the active tightening load of the belt tightening device, are compensated without here any risk for damages or failure of the toothed components arises and thus the capacity is at least not reduced. Further, rapid tightening shall be possible when the belt tightening device is triggered.